1. Field of the Invention
This invention generally relates to the protection of optical discs from illegal copying, and more particularly to optical discs provided with an illegal copying preventing function, recorders for recording signals in the original discs of the optical discs, and readout units for the optical discs.
2. Related Art
In a compact disc (CD), which is a read-only optical disc, signals have been EFM modulated and recorded as pits (or lands) of 3T to 11T long. Here, T is a 1-channel bit period, that is, the length of a pit (or xe2x80x9ca landxe2x80x9d) that regenerates a signal having a predetermined read clock length. Hereinafter in this specification, T will be used with this meaning.
To record the EFM-modulated signal as pits of integer times as long as T within a range of 3T to 11T, the duty cycle of the EFM-modulated signal is compensated. This is signal compensation considering the beam waist of a recording laser beam. That is, when an original disc is subjected to laser beam cutting in accordance with such compensated signal, the pits are formed exactly to a length of integer times as long as T within a range of 3T to 11T. For this compensation, a recorder for original discs is equipped with a duty compensator between an EFM encoder and an E/O modulator.
The illegal copying of optical discs are performed in such a way that, for example, a protective film is removed from a normally manufactured optical disc, a stamper is made by being plated with the exposed aluminum reflection layer used as an electrode, and a copied optical disc is made by injection molding or the like using this illegal stamper.
There are demands for protecting against illegal copy of read-only optical discs. Also, there are demands for identifying illegally copied optical discs. Furthermore, there are demands for providing a readout unit provided with functions of disabling the readout of illegal discs.
It is a primary object of the present invention to satisfy such demands.
The present invention is to provide a device in which micro-pits (pit length: 1T, for example) which are disabled precise copying by means of illegal copying described above and enable signals different from signals of a normal disc to be detected in case of failure in precise copying are recorded in an optical disc beforehand, determination is made whether the subject disc is a normal disc or not based on whether these micro-pits are normally detected or not, and readout of the subject disc is enabled or disabled based on such determination. The micro-pits or micro-pit strings recorded in the optical disc according to the present invention can not be precisely copied by any method which has been conventionally used for illegal copying. Therefore, it can be determined whether a loaded disc is an illegally copied disc or a normal disc based on whether the micro-pits are detected or not or based on the signals read out of the micro-pit strings are normal or not. Also, based on the results of such determination, reading of illegal discs can be disabled.
Still also, the present invention is to provide a device in which micro-variations which are disabled precise copying by means of illegal copying and enable duty variation components different from duty variation components of a normal disc to be detected in case of failure in precise copying are provided to the pit length of an optical disc beforehand, determination is made whether the subject disc is a normal disc or not based on whether such micro-variations are normally detected or not, and readout of the subject disc is enabled or disabled based on such determination. The micro-variations provided to the pit length of the optical disc according to the present invention can not be precisely copied by any method which has been conventionally used for illegal copying. Therefore, it can be determined whether a disc is an illegally copied disc or a normal disc based on whether the micro-variations of the pit length are present or not or based on whether the micro-variations of the pit length are coincided with the micro-variations which should have been recorded in the normal disc. Also, based on the results of such determination, readout of illegal discs can be disabled.